20G 



ixvEi:Ti':i;i;A'i'i': animals. 



nrck, viiicli c:in lie (i]iene<l ami cliiseil :itwill. Tliimgli tlius adapted 

 fcii- liivatliiii,!;- air diruetly, iiiai]}' of the Members (jf this group can 

 iinly live in damp or moist places, while others liabitnally live in 

 fresh water. The eommon Pond-snails are examples of these last. 

 Tlie condition of the shell varies \cry mncli. Some, snch as the 

 e(anni.in L.'ind snails (IIl'. 1 li'), have a. well-developed shell within 



Fi,^'. 1 1 



-I h,i X S irrrl 



(if thu Slii-s. (Mlt-y W Iw.ircl.) 



which Ihe anim.-d can comjiletely withdraw itself for protection. 

 (itlicrs, snch as the connnon S1u,l;s (fig. 146), have a rudimentary 

 shell which is eoni|>letely eonce.-ded \\itliin the mantle. Others are 

 entirely destitute of a shell. They all agree with tlie typical Gas- 

 terojiods in creeidng aljont on a lii'oad, flattened foot. 



( LASS III. l'rKl!l>Polj..V. 



This class is .-I veiy small one, .■ind ineluiles a nundier of miinite 

 nrriijiir Molluscs, often called " Wingeil Snails," whicli are found 

 •swimming near the surface in the o]>en ocean, far from land, and* 

 often in enoi'mous numbers. The organs c»f locomittion are two 

 wing-like fins (fig. 147) att.iclied to the sides of the head, and formed 

 by .a modification of a ])ortion of the foot. The body is usually pro- 

 tected by a synniietrical glassy shell (fig. 147, ]•), i-onsisting of two 

 ])l;ites ^niitc'd along their cilges, or in other cases foi'ming a spiral 

 (fig. 1J7, A ;iiiil ( '). In sonic, however, there is no shell, and the 

 body is (pn'te n.aked (fig. 147, 1!). The head is nidimentarv, and 

 bi'ars the mouth, which is furnished with an odontojihore. The 

 heart consists of an auricle .and \'eulricle, ami the ivspiratory org.ans 

 .•■re extremely rudiniiiitary. T]\r sexes ai'c uinted in the same indi- 

 vidual in all the /'/n-njiiHhi. 



Tlie Ptei'opod.a occur, as ali'cady said, in the o])en ocean, and they 

 ai'c fonnd in .all si'as fi'om the tro|iics to witbin the arctic circle, 

 sonii'tbiies in such mnidters as to dise(»I()ur the A\'ater for many 

 mill's. Miiiiile as they are, they constitute in high latit\ules one of 

 the staple articles of did of Ihe whale, and they themselves in turn 



